About

Xpra is 'screen for X', and more: it allows you to run X programs, usually on
a remote host and direct their display to your local machine. It also allows you
to display existing desktop sessions remotely.
Xpra is "rootless" or "seamless", and sessions can be accessed over SSH,
or password protected and encrypted over plain TCP sockets.
Xpra adapts to bandwidth constraints and is fully open-source.
For more details see here, you can find the source here.

Enhancements

This fork adds many enhancements over the original version:
better performance and platform support, hardware acceleration, multi-user support, GUI tools and configuration and options;
keyboard and clipboard fixes; cursor, sound, system tray and notifications forwarding;
IPv6 support, support for shadowing existing displays, etc; and many bug fixes.
Development is very active, so keep an eye on the news for new versions and features.

Source snapshots can be found here: xpra.org/src, the subversion repository is here: xpra.org/svn/Xpra, for more information see here.
Winswitch is a front-end for xpra (and also VNC, NX, RDP, etc),
it is available here and the binaries include xpra.

Get started with Xpra

To start an application (a simple xterm in this example) on a remote system
and display it your local machine, simply run:

xpra start ssh:SERVERHOSTNAME:100 --start-child=xterm

This is similar to ssh -X, except it is faster and you can disconnect and re-connect to the session as many times as you like. (obviously xpra must be installed on your server host)
To re-connect or to connect to this session from another machine, use the attach command (you can also use the xpra_launcher GUI):